A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms, molecules or ions of an active medium which have been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed, round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transition to a lower energy state can stimulate the emission of photons of identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of identical energy and exactly in phase. A portion of this cascade of photons is then discharged through one or more of the reflecting surfaces of the optical cavity.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices, which are also referred to as laser diodes, are typically constructed from materials such as gallium arsenide and aluminum gallium arsenide alloys. The efficiency of such lasers in converting electrical power to output radiation is relatively high and, for example, can be in excess of 40 percent.
The use of flashlamps, light-emitting diodes, laser diodes and laser diode arrays to optically pump or excite a solid lasant material is well known. Lasant materials commonly used in such solid state lasers include crystalline or glassy host materials into which an active material, such as trivalent neodymium ions, is incorporated. Detailed summaries of conventional crystalline lasant materials are set forth in CRC Handbook of Laser Science and Technology, Vol. I, M. J. Weber, Ed., CRC Press, Inc., Boca Raton, Fla., 1982, pp. 72-135 and in Laser Crystals, Vol. 14 of the Springer Series in Optical Sciences, D. L. MacAdam, Ed., Springer-Verlag, New York, N.Y., 1981. Conventional host materials for neodymium ion include glass, yttrium aluminum garnet (Y.sub.3 Al.sub.5 O.sub.12 referred to as YAG), YAlO.sub.3 (referred to as YALO) and LiYF.sub.4 (referred to as YLF). By way of example, when neodymium-doped YAG is employed as the lasant material in an optically pumped solid state laser, it is typically pumped by absorption of light having a wavelength of about 808 nm and emits light having a wavelength of 1,064 nm.
U.S. Pat. No. 3,624,545 issued to Ross on Nov. 30, 1971, describes an optically pumped solid state laser composed of a YAG rod which is side-pumped by at least one semiconductor laser diode. Similarly, U.S. Pat. No. 3,753,145 issued to Chesler on Aug. 14, 1973, discloses the use of one or more light-emitting semiconductor diodes to end pump a neodymium-doped YAG rod. The use of an array of pulsed laser diodes to end pump a solid lasant material such as neodymium-doped YAG is described in U.S. Pat. No. 3,982,201 issued to Rosenkrantz et al. on Sep. 21, 1976. Finally, D. L. Sipes, Appl. Phys. Lett, Vol. 47, No. 2, 1985, pp. 74-75, has reported that the use of a tightly focused semiconductor laser diode array to end pump a neodymium-doped YAG results in a high efficiency conversion of pumping radiation having a wavelength of 810 nm to output radiation having a wavelength of 1,064 nm.
Materials having nonlinear optical properties are well known. For example, U.S. Pat. No. 3,949,323 issued to Bierlen et al. on Apr. 6, 1976, discloses that nonlinear optical properties are possessed by materials having the formula MTiO(XO.sub.4) where M is at least one of K, Rb, Ti and NH.sub.4 ; and X is at least one of P or As, except when NH.sub.4 is present then X is only P. This generic formula includes potassium titanyl phosphate, KTiOPO.sub.4, a particularly useful nonlinear material. Other known nonlinear optical materials include, but are not limited to KH.sub.2 PO.sub.4, LiNbO.sub.3, KNbO.sub.3, .beta.-BaB-2.sub.4, Ba.sub.2 NaNb.sub.5 O.sub.15, LilO.sub.3, HlO.sub.3, KB.sub.5 O.sub.8 .cndot.4H.sub.2 O, potassium lithium niobate and urea. A review of the nonlinear optical properties of a number of different uniaxial crystals has been published in Sov. J. Quantum Electron, Vol. 7, No. 1, January 1977, pp. 1-13. Nonlinear optical materials have also been reviewed by S. Singh in the CRC Handbook of Laser Science and Technology, Vol. III, M. J. Weber, Ed., CRC Press, Inc., Boca Raton, Fla., 1986, pp. 3-228.
The nonlinear nature of the optical susceptibility of nonlinear optical materials provides a coupling mechanism between electromagnetic waves that simultaneously pass through the material and can be used to generate radiation by the interaction of these waves. As used in this application, the term "optical mixing" refers to the interaction within a nonlinear optical materials of two beams of light having frequencies w.sub.1 and w.sub.2 to produce optical radiation of a different frequency. For example, where w.sub.1 is greater than w.sub.2, this interaction can produce optical radiation at the sum-frequency, w.sub.3 =w.sub.1 +w.sub.2, and the difference-frequency, w.sub.4 =w.sub.1 -w.sub.2. These two processes are referred to as sum-frequency generation and difference-frequency generation, respectively. Up-conversion refers to the special case of sum-frequency generation where radiation of one frequency, for example w.sub.1, is much more intense than that at w.sub.2 and, accordingly, does not undergo any appreciable change in amplitude as optical mixing occurs to give optical radiation of wavelength w.sub.3. Optical mixing also includes higher order processes such as w.sub. 5 =w.sub.1 +2w.sub.2 and w.sub.6 =2w.sub.1 -2w.sub.2. For the purposes of this application, the optical radiation produced by optical mixing is generically referred to as "optical mixing radiation."
Efficient optical mixing within a nonlinear optical material is not usually possible unless the wave vectors, k.sub.1, k.sub.2, and k.sub.3, of the interacting waves satisfy the momentum conservation equation of phase-matching condition that requires: EQU k.sub.3 =k.sub.1 +k.sub.2
Satisfying this phase-matching requirement is not possible in isotropic crystals with normal dispersion because the refractive indices of the three different waves will necessarily be different as a consequence of dispersion. However, many nonlinear optical materials posses an anisotropy of refractive index which can be utilized to satisfy the phase-matching condition for a desired type of optical mixing.
Optical mixing can be carried out either within or outside of an optical cavity. If the process is carried out within an optical cavity, that cavity can be either: (a) a component of one of the sources of radiation for the process, or (b) separate from any cavity utilized as a component of any source of radiation for the process. For convenience, the use of such a source cavity will hereinafter be referred to as an intracavity process, and the use of a separate cavity will be referred to as an external cavity process. For the purposes of this application, an optical cavity or resonator refers to a volume, which is bounded at least in part by highly reflecting surfaces, wherein light of certain discrete frequencies can set up standing wave modes of low loss.
The up-conversion of infrared radiation to the visible and ultraviolet range has been extensively studied. Such studies have been primarily motivated by an interest in using this technique to permit the detection and analysis of infrared radiation by the conventional and efficient method that are available for light of higher frequency. Since the up-converted radiation carries essentially all of the information of the input infrared radiation, potential applications include infrared signal detection, infrared spectral analysis and infrared holography.
Up-conversion of infrared radiation has been reviewed by E. S. Voronim et al., Sov. Phys. Usp., Vol. 22, No. 1, pp. 26-45 (January 1979) and J. Warner, "Difference Frequency Generation and Up-Conversion" in Quantum Electronics, Vol., 1, Nonlinear Optics, Part B, H. Rabin and C. L. Tang, Ed., Academic Press, New York, pp. 703-737 (1975). A theoretical discussion of infrared detection by sum-frequency generation has also been published by D. A. Kleinman et al., J. Appl. Phys., Vol. 40, No. 2, pp. 546-566 (February 1969).
At page 34 of their previously-cited review article, E. S. Veronin et al;, describe the up-conversion of infrared radiation from a CO.sub.2 laser within the cavity of a YAG:Nd 3+ laser using proustite as the nonlinear optical material. In addition, E. Liu et al., Applied Optics, Vol. 21, No. 19, pg. 3415-3416 (Oct. 1, 1982) have reported the generation of radiation at wavelengths in the range from 252 nm to 268 nm by intracavity sum-frequency generation in a 90.degree. phase-matched temperature-tuned ammonium dihydrogen phosphate crystal, of selected output lines from an argon ion laser and the traveling wave in a rhodamine 110 ring dye laser. Further, U.S. Pat. No. 3,646,358, issued to Firester on Feb. 29, 1972 discloses the up-conversion of signal radiation from an external source within the cavity of a laser wherein the polarization of the signal beam is orthogonal to that of the pump beam which is generated within the laser cavity. Difference-frequency generation has been reviewed in the above-cited review article in Quantum Electronics, Vol. 1, at pp. 735-736 and by R. L. Aggarwal et al., in Nonlinear Infrared Generation, Y. R. Shen, Ed., Springer-Verlag, Berlin, pp. 19-38 (1977).
There is a current need for efficient, compact and reliable lasers which operate in the infrared, visible and ultraviolet portion of the spectrum and are capable of modulation rates over the range from 0 Hz to in excess of 1 GHz over a wide range of intensities. Such devices would be useful for applications which include optical storage of data, reprographics, spectroscopy and communications. For example, the storage of data on optical disks requires a source of coherent radiation which can be modulated at a rate between about 5 and about 20 MHz, and such radiation is desirably in the visible or ultraviolet portion of the spectrum in order to maximize data storage within a given area. In addition, compact coherent sources of red, green and blue light would be highly attractive for television applications requiring a high brightness source. The use of three such lasers in place of the red, green and blue electron guns of a conventional television picture tube would result in a high brightness television projector that would be useful in simulation systems and large screen television systems. Laser diodes possess all of the above-described capabilities except for one--their output is in a limited part of the infrared portion of the electromagnetic spectrum at wavelengths in the range from about 750 nm to about 1600 nm.
In this connection, U.S. Pat. No. 4,879,723 (Dixon et al.) discloses an efficient, compact source of coherent radiation in the infrared, visible and ultraviolet portion of the spectrum which is capable of modulation rates over the range from 0 Hz to exceed of 1 GHz. The radiation is produced by optical mixing within the optical cavity of a laser by introducing input radiation into the optical cavity where: (a) the cavity is that of a diode pumped solid state laser, and/or (b) input radiation is provided by a solid state device selected from the group consisting of laser diodes, laser diode arrays and diode-pumped solid-state lasers. The subject patent discloses a specific embodiment wherein a diode laser or diode laser array emits radiation having a wavelength near 810 nm which is focused and used to end pump a lasing medium. The lasing medium output radiation wavelength is 1064 nm where in this case, the medium is neodymium-doped YAG. A second laser diode or laser diode array, emitting radiation having a wavelength of 808 nm is simultaneously pumped into the laser cavity and mixed with the radiation emitting from the lasing medium in a nonlinear optical material to generate sum or difference frequencies. Preferably, the sum-frequency generation of visible blue light having a wavelength of 459 nm is the optical mixing radiation.
Along the same vein, U.S. Pat. No. 4,791,631 (Baumert et al.) discloses a process for producing a beam of coherent radiation at essentially 459 nm by mixing in a nonlinear optical crystal (KTP), two laser beams, one at 1064 nm and the other 808 nm. Specifically, intracavity radiation at 1064 nm is generated when a semiconductor diode laser beam at 808 nm is passed through a nonlinear optical crystal, KTP, situated within the cavity and then pumped into a neodymium-doped YAG laser crystal. The laser beam having the 808 nm wavelength used in the mixing can be derived either from the pump beam or a second semi-conductor diode laser whose light is coupled with the 1064 nm beam using a beam splitter.
Finally, U.S. Pat. No. 4,879,722 (Dixon et al.) also discloses a mixing process wherein optical mixing radiation is produced by optical mixing within an external optical cavity wherein at least one of the interacting beams of light is provided by a solid state device selected from the groups consisting of laser diodes, laser diode arrays and diode-pumped solid state lasers. The external optical cavity can be resonant for either or both of the input radiation to the cavity. The term "external optical cavity" in the subject patent pertains to a cavity that is separate from any cavity used as a component of any sources of radiation for the process.
While the mixing processes and devices described above in the Dixon et al. and Baumert et al. patents are effective, they are relatively inefficient when compared, for instance, to intracavity-doubled, diode-pumped solid state lasers and external resonantly doubled diode lasers. Thus there is a need for improved optical mixing devices and processes that retain the desirable properties of high speed modulation, limited tunability, and a broad range of possible output wavelengths but concomitantly possess enhanced output efficiency. Further, there is a need for devices and processes that can produce coherent radiation at 459 nm (blue) using sum frequency mixing techniques based on the use of robust crystals, i.e. KTP and Nd:YAG, which crystals can be expected to perform well in a commercial product.
It has now been discovered that when both inputs to the mixing process are resonantly enhanced, in contradistinction to the prior art teachings where only one of the inputs to the mixing process is resonantly enhanced, output efficiency of the mixing process can be substantially increased.